In focused ion beam (FIB) systems, ions are extracted from a source, formed into a beam, focused, and scanned across a substrate to form an image of a feature, to mill a feature or to deposit material from a gas ambient. As features become increasingly small, the FIB system must be optimized to provide a higher quality beam, that is, a smaller, more focused beam spot in which the distribution of current should be as compact as possible.
Several factors reduce the quality of the current distribution of the FIB. For ion columns using a liquid metal ion source (LMIS), a primary cause of reduced beam quality at low to moderate beam current is chromatic aberration. Gallium ions emitted from a liquid metal ion source have an energy distribution which is a combination of the intrinsic and particle interactions; the latter component is commonly referred to the as the Boersch affect. The former is very complicated as there are several different mechanisms to form the ions. Chromatic aberration is the result of particles of different energies being focused at different locations by the lenses in the ion column. The chromatic aberration causes the beam current distribution to vary with the energy spread (ΔE) of the ions. If the energies of the ions in an ion beam are plotted on a histogram showing the frequency of occurrence of ions at each energy value, the graph will have a peak at a “nominal” energy value, decrease rapidly for energies above and below the peak, and then taper off more slowly. The regions where the graph tapers off are known as the beam “tail.” The energy spread, ΔE, is typically measured as the “full width, half maximum,” that is, the energy between points at half the maximum peak value on either side of the peak. In a typical gallium liquid metal ion source, the energy spread in the beam having a current of 1 pA to several hundred nA is typically about 5 eV at an emission current of 1.5 to 2.5 μA from the source.
FIGS. 1A-1C are photomicrographs that show the effects of the beam energy tail on ion beam milling of photoresist. The features shown in FIGS. 1A-1C were milled using a gold-silicon ion source, with a beam current of 0.2 nA. In FIG. 1A, the beam was applied for two seconds to provide a dose of 4×1014 ions per cm2. The beam was moved in a square pattern to mill a central square 100 nm on a side. The ions in the energy tail, having energies away from the peak were deflected differently in the ion column and fell outside the square, milling the photoresist lightly out to circle 102.
In FIG. 1B, the beam was applied for 10 seconds for a total ion dose of 2×1015 ions per cm2. The relative number of ions having a particular energy value decreases as the particular energy value is farther from the nominal beam value. That is, the number of ions gets smaller as the energy value gets farther from the nominal value. As the total number of ions is increased, however, ions having energies farther from the nominal value will also increase in number. The longer the milling operation, the more the effects of ions further out in the energy tail will be seen. The circle 102 is wider in FIG. 1B than in FIG. 1A because ions further in the tail from the nominal value are having an increased effect because of their increased number. In FIG. 1C, the beam was applied for 100 seconds for a total ion dose of 2×1016 ions per cm2, and the circle 102 is even wider as the number of ions further away from the nominal energy value increases and the effects are more visible.
For e-beam imaging systems, the imaging acuity is a function of the chromatic aberrations in the focusing column, combined with contributions from the source size, diffraction, and spherical aberration. Chromatic aberration is proportional to the energy spread of the electrons, thus if an energy filter were used in an e-beam column, smaller beams could be achieved, thereby improving imaging acuity.
One type of prior art energy filter is a “chicane” dual-bend energy filter in which the charged particle beam is jogged off-axis and then back on-axis, usually by four dipole deflectors in series down the column, typically located between the upper column and the final lens. Between the second and third deflectors of the chicane, there is usually positioned either a knife-edge or a round aperture to block the passage of ions either below a nominal energy for a high pass filter or within a nominal energy range for a band pass filter, respectively. In either case, since the beam is focused to a crossover at the aperture plane by a lens above the energy filter, increased space-charge effects will inevitably occur. These effects increase both the energy spread of the beam, known as the “Boersch Effect” and the transverse spatial broadening of the beam, known as the “Loeffler Effect.”
FIG. 2 is an isometric view of a prior art chicane energy filter 200 comprising four dipole elements 202, 204, 206, and 208. For proper energy filtering, a lens (not shown here) located above the column focuses the charged particle beam into a round spot (not shown) in the plane of the aperture within aperture assembly 210. For high- or low-pass energy filtering, the aperture may be a knife-edge or slit. For bandpass energy filtering, the aperture may be either a slit or a circular hole. In either of these cases in the prior art, the beam current density will typically be very high at the aperture since the beam is focused along both the dispersion axis (vertical in this view) and perpendicular to the dispersion axis (upper right to lower left in this view). Thus, for both ion and electron beams, strong space-charge interactions may occur at and near the aperture. For ion beams, this high beam current density will also produce undesirable sputtering of the aperture for all those ions outside the pass-band of the energy filter. The four elements 202, 204, 206, and 208 are excited as dipoles to provide the required beam offset at the aperture. In this illustration, 202, 204, 206, and 208 have been implemented as octupoles, each with eight independently-excitable electrodes, which is a common practice known in the prior art to improve the electrostatic field uniformity over a wider physical aperture, thereby reducing optical aberrations. In the prior art, elements 202, 204, 206, and 208 are alternatively known to be implemented as pairs of parallel flat electrostatic electrodes or as quadrupoles.
In prior art chicane energy filters, the beam is brought to a focus at the aperture plane and those ions having energies which are not desired in the final beam are caused to impact the aperture, thereby blocking them from passing into the final lens. Due to the high beam current densities and energies (usually the final beam energy which can be up to 30-40 keV) of these blocked beams, sputtering of the aperture may be significant, sometimes resulting in sputtering holes completely through the aperture. Once this has happened, the charged particle beam system must be opened for replacement of the damaged aperture.
In certain FIB columns, the beam blanker is a single deflecting element, which means that it is not possible for the beam to be “conjugately blanked,” which would allow the beam to go on and off without any beam motion at the sample. When using a single deflecting element as a beam blanker, as the beam is deflected off-axis in order to prevent it from reaching the sample, the beam is simultaneously moved at the sample, causing the beam to expose and thereby mill areas which are not intended to be processed by the beam.
Additionally, in some current FIB columns, some particles leave the source as neutrals, or are neutralized within the column before the final lens. Since these particles are uncharged, they cannot be focused onto the target and result in a wide neutral background that may cause unwanted milling or a loss in image contrast. Therefore, it would be desirable to be able to remove this background from the beam before it enters the final lens.
The prior art chicane filters employ a round focus at the energy-filtering aperture and are negatively impacted by both space charge effects and aperture damage. Thus, there is also a need for an energy filter capable of reducing these adverse effects on imaging and milling acuity, as well as aperture lifetime.